1. Field of the Invention
The present invention relates to a so-called sheet-through type image reading device for reading an image of a document so conveyed as to move in a sub-scanning direction on one principal surface of a transparent plate at the other principal surface side of the transparent plate.
2. Description of the Related Art
When an image of a document is read by using a sheet-through type image reading device and printed out by using a printer or is captured in a personal computer and displayed on a display, a linear abnormal image sometimes appears in the image along a sub-scanning direction. This is caused when a foreign matter such as dirt, powder dust or a piece of paper sheet of a conveyed document is attached to a contact glass of the image reading device. The linear abnormal image appears on a white background base portion in a form of a black line or appears on an image portion in a form of a white line. These linear abnormal images are visually eye-catching.
As a technology to suppress occurrence of such linear abnormal image, a technology disclosed in Japanese Patent No. 3337993 is known. According to the conventional technology, a reference plate so positioned as to extend in a main scanning direction at a document reading position is read before a document is read. Then, whether or not the abnormal image is included in an obtained image of the reference plate is determined. If the abnormal image is included, a document reading position is moved along a sub-scanning direction. If an abnormal image is identified at a newly set document reading position, the document reading position is further changed. Consequently, the conventional technology suppresses occurrence of the linear abnormal image appeared on an image of the read document.
However, according to the conventional technology, an image is read at a position displaced from the most preferable and proper position in design since the document reading position is changed. Accordingly, a degree of reading sensitivity, i.e. intensity (i.e. pixel value) of an image obtained from a specified document, is deviated from the most preferable value in design. Consequently, there has been a problem that a background fogging (a phenomenon in which a mist-like coloring occurs in an area initially having a white background) appears on an image obtained by reading a document and a problem that coloring is changed slightly in a color image.
Further, many of the sheet-through type image reading devices are positioned such that a document is fed over a contact glass while bending the document back at a roller so that a document feeding-in position and a document feeding-out position are approximately on the same line. Accordingly, the image reading device is made smaller in size. Since the document is fed over the contact glass while being bent back, the document moves on the contact glass while being curved. Accordingly, when the document reading position is changed, a distance and angle of the document sheet with respect to the contact glass changes. Accordingly, a reading sensitivity is sensitively changed by the change in the document reading position so that its effect is sensitively reflected in an image quality of a document image. Especially, in the case of a document sheet having a high reflectivity such as a glossy paper sheet, the effect becomes more prominent.